Retarding camshaft timing in an engine equipped
with a dual equal camshaft timing phaser reduces the unburned
hydrocarbons (HC) and oxides of nitrogen (NO ) emitted to
the exhaust system. Apart from this positive effect to feedgas
emissions, camshaft timing can cause large air-to-fuel ratio excursions
if not coordinated with the fuel command. Large air-to-fuel
ratio excursions can reduce the catalytic converter efficiency and
effectively cancel the benefits of camshaft timing. The interaction
between the camshaft timing and the air-to-fuel ratio results in
an inherent tradeoff between reducing feedgas emissions and
maintaining high catalytic converter efficiency. By designing
and analyzing a decentralized and a multivariable controller, we
describe the design limitation associated with the decentralized
controller architecture and we demonstrate the mechanism by
which the multivariable controller alleviates the limitation.
Index Terms—Air-to-fuel ratio, emissions, internal combustion
engines, multivariate feedback control, pollution control.